The present invention relates to a method for the manufacture of sheetlike or weblike material, in particular writing paper or document paper, provided with markings detectable or becoming visually apparent in transmitted light or by the penetration of rays. The invention also relates to sheetlike or weblike material manufactured in accordance with the method. It has long been known that writing and document paper can be provided with so-called water marks. Such water marks in principle are invisible but become clearly apparent when the sheet of paper is held up against the light or light is transmitted through it in some other way. The water marks are traditionally produced by means of impressions in, or simple touching of, the strongly aqueous paper pulp distributed over the endless wire of the paper machine. The operation of producing the water marks is carried out with the help of a so-called dandy roll.
Such water marks are expensive to produce when the quantities of water-marked material to be manufactured are not very large so that it is rarely economically justifiable to provide e.g. private writing paper with special water marks.
There is a possibility, however, to produce so-called false water marks by chemical methods. These "water marks" are produced by locally modifying the optical refractive index through applying a chemical compound, often a polymerizable substance, in the desired pattern. However, it has been found that these "false water marks" in general are often visible in light other than transmitted light and that they give the impression of the paper having been marked by a "grease stain". A further possibility consists in altering the light transmitting capacity of the paper by machining portions of the paper surface so as to thin the paper in a pattern corresponding to the text or images. However, such a mechanical processing of the paper surface gives clearly visible traces which are distinctly apparent even without any light transmission. The abovementioned methods are subject to inconveniences which are eliminated by the present invention which is characterized in that the desired markings of the water mark type are arranged between two sheets or webs joined to one another.